Heretofore images of high quality have been produced by thermal printers. In a typical thermal printer an image is formed in three passes. First a colorant patch having color such as yellow is placed in transfer relationship with a receiver and then the colorant patch is heated in a pattern corresponding to the yellow portion of an image to be completed. Thereafter, cyan and magenta portions of the image are formed in a similar fashion. The completed color image on the receiver is continuous tone and in many cases can rival photographic quality.
In one type of thermal printer which prints colored images, a donor contains a repeating series of spaced frames of different colored heat transferable colorants. Thermal colorant transfer printers offer the advantage of true “continuous tone” density transfer. This result is obtained by varying the energy applied to each heating element, yielding a variable density image pixel in the receiver. The donor is disposed between a receiver, such as coated paper, and a print head formed of, for example, a plurality of individual heating resistors. When a particular heating resistor is energized, it produces heat and causes colorant from the donor to transfer to the receiver. The density or darkness of the printed color colorant is a function of the energy delivered from the heating element to the donor.
Under common circumstances after an image is printed, a protective layer of material is coated in order to prevent damage to the image. Commonly-assigned U.S. Pat. No. 5,369,077 teaches that silicone block copolymers are added to the receiver and receiver overcoat to prevent sticking to the colorant patch. Though this effectively protects the image it hurts the ability to affix information carried by a water soluble inks or pigments, for example a rubber stamp mark. Rubber or polymer stamp marks normally consist of water soluble inks or pigments. Images produced using a thermal printing process provide a convenient method for creating images for use as identification, for example as passport and visa pictures and small pictures that are attached to school, job or club applications. When pictures are used for identification purposes, the pictures and the documents to which they are attached may require some type of official stamp. In most cases the stamp is an official seal made of rubber. The rubber stamp is used to apply the official seal to the document and picture. The marking medium is a water soluble ink or pigment that is readily absorbed by the material to which it is applied. In the case of thermal media during the printing process a protective transparent layer is coated that is water resistant thus making the adhesion of a rubber stamp impossible. Commonly-assigned U.S. Pat. No. 5,614,464 teaches the addition of perfluorinated alkyl sulfonamide ester copolymers to improve receiver writeability. This coating on the other hand may help the adherence of water based inks or dyes.